This invention relates to methods for manufacturing electrical humidity detectors, and more particularly to techniques for calibrating such detectors.
Humidity detectors are incorporated into heating, ventilation and air conditioning (HVAC) systems which control the environment within a building. These detectors emit an electrical signal which indicates the level of relative humidity in the zone of the building in which the detector is located. A typical detector comprises a humidity sensor which has a material with an electrical characteristic that varies with variation of humidity to which the sensor is exposed. The sensor is connected to an electrical circuit, often referred to as a transmitter, which normalizes the signal from the sensor and compensates for differences between the actual and ideal electrical characteristics of the sensor.
U.S. Pat. No. 5,408,381 describes one type of a humidity sensor. In this sensor, conductive layers are bounded to opposite faces of a film core. The core is made of a polyimide that has a dielectric constant that varies in a substantially linear manner with humidity. Thus, the capacitance of the sensor changes with the ambient humidity. Because of variations in the polyimide core and manufacturing tolerances, the specific relationship of the capacitance to humidity differs from sensor to sensor and typically differs from the ideal relationship characteristic.
As a consequence of the difference between the actual sensor capacitance and the ideal capacitance, the transmitter circuit to which the sensor is connected provides a mechanism for altering the electrical signal representing humidity in order to compensate for such differences. Conventional transmitters are manufactured with two signal adjustments, offset and gain (or sensitivity). If the output signal from the transmitter is not a linear function of the humidity, a third adjustment may be provided to make the transfer function fit a desired linear characteristic.
During the manufacture of the humidity detector, the electrical signal characteristics are calibrated to match the desired characteristics over the range of relative humidity (RH) with which the detector is intended to operate (e.g. 10-80%). The calibration is performed by adjusting the settings at two widely separated humidity levels within the intended operating range. The procedure requires that the assembled detector be placed within a test chamber that exposes the sensor to an accurately maintained first humidity level for a long enough period for the performance to stabilize. Once the sensor has stabilized, the electrical signal produced by the detector is compared to the ideal signal level indicative of the first humidity level and the offset of the transmitter circuit is adjusted so that the actual signal level matches the ideal.
The gain of the transmitter circuit then is adjusted at a second known humidity level. Because it may take an hour or two for the sensor to stabilize at each humidity level, the detector calibration process is relatively time consuming.